This invention relates to a radio receiver including a phase-locked loop frequency synthesizer as a local oscillator, and more particularly to a muting arrangement of such radio receiver.
As is generally known, a phase-locked loop frequency synthesizer is comprised of a voltage-controlled oscillator (VCO), a divide-by-N counter to divide the output frequency of VCO by a factor of N, a phase comparator for comparing in phase a reference frequency signal with the output signal of the divide-by-N counter, and a loop filter coupling VCO with a control voltage signal with a magnitude corresponding to the phase difference between the input signals of the phase comparator. In the stable state of the phase-locked loop, that is, where the two input signals of the phase comparator coincide in phase with each other, the output frequency of VCO is equal to a product of the reference frequency and the dividing factor of the phase-locked loop. Therefore, using a stable reference frequency source, it is possible to generate an exact local oscillation frequency which permits, in turn, exact tuning of the receiver. By changing the dividing factor N of the phase-locked loop in integral steps, VCO has its output frequency varied in steps at predetermined frequency spaces (the reference frequency), thereby scanning channels within a desired received frequency range.
In such phase-locked loop frequency synthesizer, when the dividing factor N is changed it takes some period of time for the synthesizer to lock to an output frequency corresponding to a newly designated dividing factor N, and noises and/or audio signals with distortion and insufficient separation may be produced during such period of time. In order to prevent production of interstation noises, many of conventional radio receivers are provided with a mute circuit for interrupting a signal transmission path when no incoming input signal is received. Such mute circuit is so arranged as to interrupt the signal transmission path after detecting a state in which the signal input level is lowered. Accordingly, if such mute circuit is applied to a radio receiver with a phase-locked loop frequency synthesizer, there will be sounded noises and/or audio signals of the deteriorated quality from a time when the phase-locked loop is shifted from the stable state to the unstable state until a time when the signal level is actually lowered to interrupt the signal transmission path.